1. Field of the Invention
The present invention relates to a rechargeable battery. More particularly, the present invention relates to a rechargeable battery including a positive temperature coefficient (PTC) device that includes a cover member arranged on a surface of the PTC body.
2. Discussion of the Background
A rechargeable battery may be reused by repeating charging and discharging operations. Rechargeable batteries may be used in advanced electronic appliances, such as cellular phones, notebook computers, and camcorders.
Lithium rechargeable batteries may provide an operational voltage of about 3.6 V, which is about three times higher than that of Ni—Cd batteries or Ni—MH batteries. In addition, lithium rechargeable batteries may have a high energy density per unit weight, and are therefore extensively used in advanced electronic technology fields.
Lithium rechargeable batteries may use lithium-based oxides as positive electrode active materials and carbon as negative electrode active materials. Lithium rechargeable batteries may be fabricated with various shapes, such as cylinders, rectangles, and pouches.
Rechargeable batteries may include a bare cell, a protective circuit module including protective circuits, a PTC device aligned between the protective circuit module and the bare cell, and molding resin arranged in the space formed between the protective circuit module and the bare cell. The bare cell may include a can, an electrode assembly accommodated in the can, and a cap assembly coupled with the can.
In addition, safety devices, such as a PTC device, a thermal fuse, and a protective circuit module, may be installed at an outer portion of the can. The safety devices may be connected to the positive and negative electrode terminals of the bare cell. The safety devices may shut off the current in the battery if the voltage of the rechargeable battery suddenly rises due to increased temperature, overcharging, or over-discharging the battery.
FIG. 1 is a partial perspective view illustrating a rechargeable battery equipped with a conventional PTC device 170.
A PTC device 170 is a reversible safety device that is capable of stopping or reducing the current that flows through it when the internal temperature of the rechargeable battery rises above a predetermined temperature due to a malfunction of the rechargeable battery.
Referring to FIG. 1, the conventional PTC device 170 of the rechargeable battery may include a PTC body 174 having a cylindrical structure with flat upper and lower surfaces, a first conductive section 172 coupled with an upper surface of the PTC body 174 and coupled with a negative or positive electrode, and a second conductive section 176 coupled with a lower surface of the PTC body 174 and coupled with a terminal 220 that is used to make contact with an external device. An insulating member 185 may be interposed between and the PTC device 170 and the cap plate 110 to prevent a short circuit between the can 11 and the PTC device 170 because the can 11 may have a polarity opposite to that of the PTC device 170.
The PTC body 174 may be made of conductive particles distributed into crystalline polymer. Current may therefore flow through the PTC body 174 so long as the internal temperature of the rechargeable battery remains less than a predetermined temperature. If the internal temperature of the rechargeable battery exceeds the predetermined temperature, the PTC body 174 will expand to increase the distance between the conductive particles and thus stop or reduce the current that flows through it. This may stop the battery from exploding or being otherwise damaged. In addition, if the internal temperature of the rechargeable battery drops below the predetermined temperature, the crystalline polymer shrinks, so the conductive particles combine together and the current is able to flow through the PTC body 174 again.
However, a conventional PTC device 170 may be damaged during the manufacturing process when hot-melt resin, such as an epoxy-molding compound, is injected into the space between the protective circuit module and the cap plate. When the hot-melt resin cools, the PTC body 174 may shrink and the expansion of the PTC body 174 may be hindered due to the hard resin encasing the PTC body 174. Therefore, the PTC device 170 may not stop or reduce the current at the predetermined temperature. The battery may therefore continue to operate even when the internal temperature reaches dangerous levels.
Furthermore, if the PTC body 174 is cooled after directly contacting the hot-melt resin at a temperature of about 200° C. , the trip phenomenon may occur in the PTC body 174, thereby causing the conductive articles uniformly distributed in the PTC body 174 to be partially biased and may thus result in resistance dispersion. The PTC device 170 may then operate at a temperature that is different form the original predetermined temperature.